Testing a printhead

ABSTRACT

A system and method for testing printheads is disclosed. The system comprises an optical sensor mounted on a movable carriage. The optical sensor is moved past a nozzle to be tested on the printhead while the nozzle ejects ink. The output signal of the optical sensor can be used to determine when the trajectory of the ejected ink is improper.

BACKGROUND

Inkjet printers print images onto media by ejecting ink drops from aprinthead. Each printhead typically has an array of nozzles that ejectthe ink drops onto the media. When a nozzle is plugged or incorrectlyejects the ink drops to a different position on the media, the imagequality of the printed output may degrade. With the increase in thenumber of nozzles per printhead, detecting improperly functioningnozzles has become more difficult.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example printer.

FIG. 2 is an example block diagram of the processor 102 coupled tomemory 104.

FIG. 3 is an isometric view of an example optical sensor system 300.

FIG. 4 is a flow chart for an example method of testing a printhead.

FIG. 5A is an example top view of light beam 342 with ink drops 340Ahaving a proper trajectory.

FIG. 5B is an example top view of light beam 342 with ink drops 340Bhaving an improper trajectory.

FIG. 6A is an example cross section of the light beam in FIG. 5A atdifferent times during the printhead test.

FIG. 6B is an example cross section of the light beam in FIG. 5B atdifferent times during the printhead test.

FIG. 7A is a graph of the light detector 334 output for the ink drops340A shown in FIGS. 5A and 6A.

FIG. 7B is a graph of the light detector 334 output for the ink drops340B shown in FIGS. 5B and 6B.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of an example printer. Printer comprises aprocessor 102, memory 104, input/output (I/O) module 106, print engine108 and controller 110 all coupled together on bus 112. In some examplesprinter may also have a display, a user interface module, an inputdevice, and the like, but these items are not shown for clarity.Processor 102 may comprise a central processing unit (CPU), amicro-processor, an application specific integrated circuit (ASIC), or acombination of these devices. Memory 104 may comprise volatile memory,non-volatile memory, and a storage device. Memory 104 is anon-transitory computer readable medium. Examples of non-volatile memoryinclude, but are not limited to, electrically erasable programmable readonly memory (EEPROM) and read only memory (ROM). Examples of volatilememory include, but are not limited to, static random access memory(SRAM), and dynamic random access memory (DRAM). Examples of storagedevices include, but are not limited to, hard disk drives, compact discdrives, digital versatile disc drives, optical drives, and flash memorydevices.

I/O module 106 is used to couple printer to other devices, for examplethe Internet or a computer. Print engine 108 may comprise a media supplysystem, one or more printheads, an optical sensor system to measure inkejected by the printheads, an ink supply system, and the like. Printerhas code, typically called firmware, stored in the memory 104. Thefirmware is stored as computer readable instructions in thenon-transitory computer readable medium (i.e. the memory 104). Processor102 generally retrieves and executes the instructions stored in thenon-transitory computer-readable medium to operate the printer. In oneexample, processor executes code that directs controller 110 to test theprintheads.

FIG. 2 is an example block diagram of the processor 102 coupled tomemory 104. Memory 104 contains firmware 220. Firmware 220 contains aprinthead test module 224. The processor 102 executes the code inprinthead test module 224 to direct controller 110 to test theprinthead. In one example, the printhead test module causes the printerto check each nozzle in the printheads to determine if ink drops areejected from the nozzle in the proper trajectory.

FIG. 3 is an isometric view of an example optical sensor system 300.Optical sensor system 300 comprises a movable carriage 330, a lightemitter 332, a light detector 334, and a printhead 336. The lightemitter 332 and light detector 334 are mounted on the carriage andaligned such that the beam of light 342 emitted from light emitter 332is received by light detector 334. In some examples the light beam 342may have a circular cross section. In other examples the light beam mayhave an oval or oblong cross section. Carriage 330 moves along theX-axis underneath printhead 336. In some examples the direction thecarriage moves corresponds to the direction the paper moves duringprinting. In other examples the carriage moves in a directionperpendicular to the direction the paper moves during printing. Therelative position of the carriage with respect to the printhead may bedetermined by a linear or angular sensor coupled to the carriage motionsystem or may be determined using motor control information.

Printhead 336 comprises a plurality of nozzles. Nozzle 338 is shownejecting a stream of ink drops 340. The nozzles in FIG. 3 are shownspaced apart by a large distance for clarity. In actual practice theremay be as many as 1200 nozzles per inch along the printhead and theremay be multiple rows of nozzles. Printhead 336 is typically coupled to aprinter using a printhead mount (not shown) that locates the printheadin the printer. In some examples there may be multiple printheadsmounted in a printer. The printhead may be permanently mounted in theprinted by the printhead mount, or may be replacable.

As carriage moves underneath printhead 336 the stream of ink drops 340intersect with light beam 342. When the stream of ink drops 340intersect with light beam 342 the amount of light received by lightdetector 334 changes. The change in the signal from light detector 334can be used to determine if the stream of ink drops 340 travel straightdown (along the Z-axis) or if the stream of ink drops 340 travel at someangle with respect to the Z-axis.

FIG. 4 is a flow chart for an example method of testing a printhead. Atstep 440 the carriage is positioned such that the beam of light 342 ison one side of a nozzle to be checked such that ink ejected by thenozzle will not intersect with the beam of light 342. At step 442 thecarnage is moved towards the nozzle while ink is ejected from thenozzle. In some examples the carriage moves at a constant speed between10 inches per second and 0.1 inches per second, for example 1 inch persecond. In one example the beam is between 0.05 and 1 inch in diameter,for example 0.1 inch in diameter. The carriage will be moved until thelight beam 342 is on the other side of the nozzle to be checked suchthat ink ejected by the nozzle will not intersect with the beam of light342.

In one example a continuous stream of ink drops is ejected from thenozzle while the carnage is moving. In other examples burst of ink dropsare ejected from the nozzle. The burst may contain between 4-20 dropsper burst and each drop may be ejected with between a 40 to 170microseconds (fire frequency between 6 KHz to 24 kHz) gap between drops.In one example there is between a 100 to 2,000 microsecond space or gapbetween bursts. In some examples the carriage may not be moved at aconstant speed. Instead the carriage may be moved by a small amountbetween each burst of drops. The number of drops per burst, the spacingbetween drops, the spacing between burst and the carriage movement ratemay be varied depending of the system measurement speed, the aerosoldispersion rate, the ink color or opaqueness and the like.

At step 444 the signal from the light detector 334 is analyzed todetermine if the ink ejected by the nozzle has the proper trajectory.Ink drops that travel straight down along the Z-axis from the print head(i.e. perpendicular to the printhead) have the proper trajectory. Inkdrops that travel in a path that forms an angle with respect to theZ-axis have an improper trajectory. This method may be repeated for eachnozzle in the printhead. When a nozzle is detected ejecting ink with animproper trajectory, the nozzle may be inactivated and replace with adifferent nozzle, depending on the magnitude of the improper trajectory.In some examples a nozzle may not be ejecting ink at all. This can alsobe determined by analyzing the signal from the light detector 334.

FIG. 5A is an example top view of light beam 342 with ink drops 340Ahaving a proper trajectory. FIG. 5A shows carriage 330 with lightemitter 332 and light detector 334 mounted on the carriage 330. Carriageis moving in the X-axis as shown by arrow 550. Ink drops 340A have atrajectory perpendicular to the printhead (see FIG. 3) and parallel withthe Z-axis. Ink drops 340A are shown in a position in the middle oflight beam 342 at a time T_(m) in the middle of the printhead test. Thesize of the ink drops has been increased dramatically for clarity.

FIG. 5B is an example top view of light beam 342 with ink drops 340Bhaving an improper trajectory. The ink drops in FIG. 5A travel in atrajectory that is not perpendicular to the printhead. The trajectory isnot parallel with the Z-axis, but makes an angle with the Z-axis. Theink drops in FIG. 5B are offset in both the X-axis and the Y-axis. Onlythe offset in the X-axis can be determined by looking at the output ofthe light sensor 334. The offset in the Y-axis cannot be detected by thesignal from the light senor 334.

FIG. 6A is an example cross section of the light beam in FIG. 5A atdifferent times during the printhead test. The vertical axis correspondsto the Z-axis in FIG. 5A. The horizontal axis corresponds to the X-axisin FIG. 5A at different times during a printhead test. At time T₀ thelight beam 342 is positioned to the left of the burst of ink drops 340A.At time T₁ the light beam has moved to the right such that the burst ofink drops just intersect with the right or leading edge of the lightbeam. At time T_(m) the burst of ink drops pass through the middle ofthe light beam. Time T_(m) corresponds to the time shown in FIG. 5A whenthe ink drops pass through the middle of the light beam. At time Ti thelight beam has moved until the burst of ink drops just intersect theleft or trailing edge of the light beam.

FIG. 6B is an example cross section of the light beam in FIG. 5B atdifferent times during the printhead test. The vertical axis correspondsto the Z-axis in FIG. 5B. The horizontal axis corresponds to the X-axisin FIG. 5B at different times during the printhead test. The ink dropsin FIG. 6B do not follow a trajectory that is parallel with the Z-axis.At time T₀ the light beam 342 is positioned to the left of the burst ofink drops 340B. At time T₁ the light beam has moved to the right suchthat the burst of ink drops should be intersecting with the right orleading edge of the light beam. But because the trajectory of the inkdrops is angled away from the light beam, the ink drops do not yetintersect with the leading edge of the light beam. At time T_(m) theburst of ink drops should pass through the middle of the light beam. Inthis example the ink drops do pass through the light beam, but don'tpass through the middle of the light beam. At time Ti the light beam hasmoved until the burst of ink drops just should just intersect the leftor trailing edge of the light beam. Because the trajectory of the inkdrops is angled towards the direction of carriage motion, the ink dropsstill intersect with more of the light beam than it should.

The output signal from the light detector 334 is the response fromconsecutive bursts of ink drops intersecting with the light beam whenthe light beam is in different positions with respect to the nozzle.When the nozzle is firing with a proper trajectory, the signal outputwould be nearly the same for each light beam position. The signal willbe smaller when the burst of ink drops is at either edge of the lightbeam but the signal pattern will be the same. When the ink trajectory isnot correct, the burst of ink drops are detected at different positionsof the light beam and have a different pattern.

FIG. 7A is a graph of the light detector 334 output for the ink drops340A shown in FIGS. 5A and 6A. The vertical axis is the signal responseof the light detector 334. The horizontal axis is the position of thelight beam with respect to the nozzle location at different times duringthe printhead test. Line 770A is the output of the light detector 334 atdifferent times/positions during the printhead test. At time T₀ thelight beam 342 is positioned to the left of the burst of ink drops 340Aand the signal is at a low background level. At time T₁ the light beamhas moved to the right such that the burst of ink drops just intersectwith the right or leading edge of the light beam causing the signal toincrease sharply. At time T_(m) the burst of ink drops pass through themiddle of the light beam where the signal has flattened out at a maximumlevel. At time Ti the light beam has moved until the burst of ink dropsjust intersect the left or trailing edge of the light beam and thesignal falls rapidly back to the low background level.

FIG. 7B is a graph of the light detector 334 output for the ink drops340B shown in FIGS. 5B and 6B. The vertical axis is the signal responseof the light detector 334. The horizontal axis is the position of thelight beam with respect to the nozzle location at different times duringthe printhead test. Line 770B is the output of the light detector 334 atdifferent times/positions during the printhead test. At time T₀ thelight beam 342 is positioned to the left of the burst of ink drops 340Band the signal is at a low background level. At time T₁ the light beamhas moved to the right such that the burst of ink drops should beintersecting with the right or leading edge of the light beam.

In this case because the trajectory of the ink drop is improper thesignal is still at the low background level at time T₁ indicating thatthe ink drops have not yet intersected with the light beam. The signal770B does not start increasing until time T_(x). Therefore the ink dropsdo not start intersecting with the light beam until the beam has movedto a position corresponding to time T_(x). Using the known carriagemovement rate, the light beam diameter and the relative location of thecarriage with respect to the nozzle under test, the offset and directionof offset in the X-axis of the ink trajectory can be estimated. At timeT_(m) the burst of ink drops should be passing through the middle of thelight beam. The signal has reached a maximum level at time T_(m) but theshape of the output curve does not have a nice flat maximum level. Attime Ti the light beam should have moved until the burst of ink dropsjust intersect the left edge of the light beam. However because thetrajectory of the ink drop is improper the signal is still high and hasnot started dropping back to the low background level.

Using the shape of the light detector output curve and the position whenthe signal increases and decreases, the trajectory of the ink drops fora given nozzle can be determined. In one example the firmware inside theprinter may run the printhead tests. In other examples an externalcomputer coupled to the printer may run the printhead test.

What is claimed is:
 1. A method of testing a printhead, comprising:positioning an optical sensor on one side of at least one nozzle in theprinthead such that a leading edge of a light beam from the opticalsensor is spaced apart from the nozzle location; moving the opticalsensor towards the at least one nozzle in a direction perpendicular withthe light beam while the optical sensor is taking measurements;repeatedly ejecting ink drops from the at least one nozzle into themoving light beam; moving the optical sensor past the nozzle locationwhile the optical sensor is taking measurements such that a trailingedge of the light beam is on the other side of the nozzle location andis spaced apart from the nozzle location; analyzing the optical sensormeasurements to determine the ejected ink trajectory.
 2. The method ofclaim 1, wherein the optical sensor is moved at a constant velocity. 3.The method of claim 1, wherein the ink is ejected from the at least onenozzle in bursts with a gap between the bursts.
 4. The method of claim3, wherein the bursts comprise between 4 and 20 drops.
 5. The method ofclaim 3, wherein the gap between the bursts is between 100 and 2000microseconds.
 6. The method of claim 1, wherein optical sensor moves ina direction parallel with a printing direction for the printhead.
 7. Themethod of claim 1, wherein the nozzle is disabled when the ejected inktrajectory is improper.
 8. The method of claim 1, wherein the light beamhas a diameter between 0.05 and 1 inches.
 9. A printer, comprising atleast one printhead mount to mount a printhead, the printhead having aplurality of nozzles to eject ink onto media; an optical sensor mountedon a movable carriage, the optical sensor comprising a light source anda light detector, the light source emitting a light beam towards thelight detector; the optical carriage movable between a first positionand a second position such that a full width of the light beam passesunderneath the plurality of nozzles when the carriage is moved from thefirst position to the second position; a processor coupled to memory,the memory comprising computer readable instructions that, when executedby the processor, cause the printer to test a printhead mounted in theat least one printhead mount by: ejecting ink from at least one of theplurality of nozzles while moving the carriage from the first positionto the second position and while the optical sensor is takingmeasurements; analyzing an output from the light detector to determinethe ejected ink trajectory.
 10. The printer of claim 9, wherein thecarriage is moved at a constant velocity.
 11. The printer of claim 9,wherein the ink is ejected from the at least one of the plurality ofnozzles in bursts with a gap between the bursts.
 12. The printer ofclaim 11, wherein the bursts comprise between 4 and 20 drops.
 13. Theprinter of claim 9, wherein the carriage moves in a direction parallelwith a printing direction for the printer.
 14. The printer of claim 9,wherein the nozzle is disabled when the ejected ink trajectory isimproper.
 15. The printer of claim 9, wherein the light beam has adiameter between 0.5 and 1 inches.
 16. A printer, comprising at leastone printhead having a plurality of nozzles to eject ink onto media; anoptical sensor, the optical sensor comprising a light source and a lightdetector, the light source emitting a light beam towards the lightdetector; a carriage movable between a first position and a secondposition such that a full width of the light beam passes through andperpendicular to a line of fire for each of the plurality of nozzles; aprocessor coupled to memory, the memory comprising computer readableinstructions that, when executed by the processor, cause the printer totest at least one of the nozzles by: repeatedly ejecting ink from anozzle under test into the light beam while the light beam is movingrelative to and across the line of fire of the nozzle under test andwhile the optical sensor is taking measurements; and analyzing an outputfrom the light detector to determine whether the ejected ink from thenozzle under test is following a correct trajectory.
 17. The printer ofclaim 16, wherein the carriage supports the optical sensor to move theoptical sensor through the line of fire of the nozzles.
 18. The printerof claim 16, wherein the processor is programmed to control the nozzleunder test to eject a continuous stream of ink drops while the lightbeam is passing through the nozzle's line of fire.
 19. The printer ofclaim 16, wherein the processor is programmed to control the nozzleunder test to eject bursts of ink drops with gaps in between the burstswhile the light beam is passing through the nozzle's line of fire. 20.The printer of claim 16, wherein the processor is programmed to analyzea shape of a light detector output curve to determine whether the nozzleunder test is ejecting ink on the correct trajectory.